That makes sense. but if that's the actual thickness, then It seems that armor is too light. I know very little about ships but I would imagine adding an inch onto the main belt of a large battleship would weigh more then a couple hundred tons at the best quality. this is based off no real knowledge, just my intuition, so correct me if I'm wrong.
PS: after doing some math the game seems pretty accurate. assuming a sq foot of 1 in armor weighs about 40 lbs(a measurement I found on the internet), then 2910 ton main belt could be 18 in thick, 5 yards tall and 179 yards long which makes sense.